In high performance audio amplifiers which utilize typical feedback networks, such as the prior art amplifier 10 shown in FIG. 1, the ratio of resistor 20 to resistor 18, or R1/R2, is such that severe loading problems may occur at the amplifier output 14. This is especially true when the audio circuitry utilized in conjunction with the audio amplifier is of high output impedance, as is the case of high voltage amplifiers utilizing discrete components. These loading problems can cause distortion and compression of the dynamic range for audio amplification.
According to the present invention, an audio amplifier circuit is shown which removes such loading problems from the amplifier output. The audio amplifier circuit comprises a discrete component amplifier having a non-inverting input, an inverting input and an output with an associated load thereof. An amplified negative feedback network substantially removes the load from the output of the discrete component amplifier. The amplified negative feedback network includes a feedback amplifier having a non-inverting input, an inverting input and an output, a first resistor network and a second resistor network and a further feedback network. The first resistor network includes a first resistor connected between the output of the discrete component amplifier and the non-inverting input of the feedback amplifier and a second resistor connected between the non-inverting input of the feedback amplifier and a signal common. The second resistor network includes a third resistor connected between the output of the feedback amplifier and the inverting input of the discrete component amplifier and a fourth resistor connected between the inverting input of the discrete component amplifier and signal common. The further feedback network provides local negative feedback for the feedback amplifier. The further feedback network comprises a fifth resistor connected between the inverting input of the feedback amplifier and signal common and a sixth resistor connected between the inverting input of the feedback amplifier and the output of the feedback amplifier.
A method for providing feedback for an audio amplifier composed of discrete components is also shown. The audio amplifier comprises a non-inverting input, an inverting input and an output with an associated load. An amplified feedback network is connected between the output of the audio amplifier and inverting input of the audio amplifier. The amplified feedback network includes a feedback amplifier having an inverting input, a non-inverting input, and an output. A first resistor and a second resistor are connected between the output of the audio amplifier and a signal common as a voltage divider such that the non-inverting input of the feedback amplifier is connected therebetween. A third resistor and a fourth resistor are connected between the output of the feedback amplifier and the signal common such that the inverting input of the audio amplifier is connected therebetween. A fifth resistor and sixth resistor are connected between the output of the feedback amplifier and signal common such that the inverting input of the feedback amplifier is connected therebetween. The first resistor and second resistor are selected to produce an impedance significantly greater than the output impedance of the audio amplifier such that the load associated with the output of the audio amplifier is substantially removed.